I downgraded my iPad Air 2020 from iPadOS 15 to 14.8 a few days ago by connecting it my MacBook Pro 2014 running Mojave. It told me I needed the update to connect, I selected to install the update, then it connected without issue. If it does that, then it should allow backups onto Mojave, but I use iCloud backups so I didn’t test backups via iTunes on Mojave. So to reiterate, iPadOS 15 was able to connect to iTunes on Mojave.